16 week ultrasound 99% good!

Cookie is Good! Heart rate 135, measuring at between 16 and 16 1/2 weeks for the different measurements. and my cervix is still long. The one thing that is worrying me is that the placenta is low and posterior " not touching but very close" were the words of the sonographer, I have to see my GP on wed for the official ultrasound report.  Why don't i feel more relaxed about this pregnancy, I've just seen Cookies Heartbeat and watched little fingers and toes moving, So why aren't I excited. I'm definitely relieved, but not excited.